The effect of changes in intraocular pressure on corneal curvature after radial keratotomy in the rabbit eye

Standard radial keratotomy was performed in both eyes of 18 rabbits. Intraocular pressure (IOP) was raised by injection of saline solution into the vitreous cavity at different times after surgery: 8 eyes at one, two, and six weeks and 12 eyes at four months. Keratometric readings at 10, 20, 40, 60, and 80 mmHg were recorded by three independent observers in a masked fashion. The same procedure was performed in eight eyes that had no surgery, as controls. Increments in IOP correlated with significant reductions in corneal power at all times after surgery and for all IOP values in the operated eyes. No changes were observed in the control eyes. The results indicate that variations in IOP, even within physiological limits, may cause changes in corneal curvature after radial keratotomy, and that patients with wide fluctuations in IOP could present a higher risk for postoperative fluctuation in visual acuity after this refractive procedure.

Epikeratophakia for myopia correction

Epikeratophakia is based on the principles of the Barraquer refractive procedures, with modifications that simplify the surgical technique and eliminate the use of the microkeratome by placing the donor corneal tissue lens on the anterior surface of the cornea. Procedures developed to permit freeze-drying the preshaped lens for storage enable these lenses to be obtained from a central source, freeing the surgeon from the complexities of the computer and the cryolathe. The correction of theoretically unlimited amounts of myopia is possible with these lenses. In 12 eyes that underwent the final epikeratophakia procedure, the average desired correction achieved was 98%.

Acanthamoeba keratitis associated with soft contact lenses

Three patients (a 13-year-old girl, a 25-year-old man, and a 22-year-old woman) who used daily-wear soft contact lenses, sterilized with saline made from distilled water and salt tablets, developed Acanthamoeba keratitis. Acanthamoeba was cultured from the contact lens solution of one patient. This patient, in whom the diagnosis was made by corneal biopsy early in the clinical course, was successfully treated with topical neomycin-polymyxin, miconazole, and propamidine isethionate. The other two patients underwent penetrating keratoplasty. One of these patients, who received a graft early in the clinical course, developed a recurrence of disease in the graft, whereas the other, who received the graft 18 months after the initial symptoms, has maintained a clear corneal transplant with useful vision.

Cataract induction in rabbits with the Nd-YAG laser

The study of occlusion amblyopia and its therapy has involved animal models of stimulus deprivation achieved by various means, none of which closely simulates human congenital cataract. The authors used the Nd-YAG laser as a means of inducing cataracts in rabbit eyes. Twenty rabbit eyes were treated at various frequency and power settings. High energy YAG laser pulses of 10 mJ produced cataracts that at first resembled large bubbles. Over several days, these bubbles coalesced into smaller opacities. With multiple treatments (usually 150-200 pulses), the authors could create a cataract of a specific size and position. Six months later, 100% of these laser-treated rabbit eyes showed persistent cataracts that resembled human congenital cataracts. It appears that the YAG laser can provide a reproducible and reliable method of inducing specific types of cataracts in animal eyes without damaging other ocular structures.

Results of the prospective evaluation of radial keratotomy (PERK) study one year after surgery

The Prospective Evaluation of Radial Keratotomy (PERK) study is a nine-center, self-controlled clinical trial of a standardized technique of radial keratotomy in 435 patients who had physiologic myopia with a preoperative refraction between -2.00 and -8.00 diopters. The surgical technique consisted of eight incisions using a diamond micrometer knife with blade length determined by intraoperative ultrasonic pachymetry and the diameter of central clear zone determined by preoperative refraction. At one year after surgery, myopia was reduced in all eyes; 60% were within +/- 1.00 diopter of emmetropia; 30% were undercorrected and 10% were overcorrected by more than 1.00 diopter (range of refraction, -4.25 to +3.38 D). Uncorrected visual acuity was 20/40 or better in 78% of eyes. The operation was most effective in eyes with a refraction between -2.00 and -4.25 diopters. Thirteen percent of patients lost one or two Snellen lines of best corrected visual acuity. However, all but three eyes could be corrected to 20/20. Ten percent of patients increased astigmatism more than 1.00 diopter. Disabling glare was not detected with a clinical glare tester, but three patients reduced their driving at night because of glare. Between six months and one year, the refraction changed by greater than 0.50 diopters in 19% of eyes.

Electron Microscopic Study of Intrastromal Hydrogel Implants in Primates

Three corneas with intrastromal hydrogel implants (surfilcon A) were removed from Green monkey eyes by penetrating keratoplasty and examined by light microscopy, as well as scanning and transmission electron microscopy, in order to assess the tolerance of the primate cornea for this type of synthetic plastic. Placement of these implants in the posterior cornea appears to increase the amount of protrusion into the anterior chamber, which can also be seen clinically on slit-lamp biomicroscopy. Some physiological stress to the cornea was indicated by abnormalities above and below the implant, including thinned epithelium, irregular cell shapes, and vacuolations. It appears that a basement-membrane-like material is produced by keratocytes adjacent to the implant-stroma interface, and that this material fills the spaces and provides some physical continuity between the plastic and the corneal tissue. No inflammatory reaction was seen around the implants, but further long-term studies are necessary to ensure compatibility between the cornea and the implant.

Epikeratophakia in children

Epikeratophakia was performed in 61 children for the correction of aphakia after the removal of unilateral congenital or traumatic cataracts; 51 patients (54 grafts) have more than 6 months follow-up. In the first 27 grafts, 8 (30%) failed, but in the last 27 grafts, only 2 (7%) failed, largely due to improved surgical and tissue handling techniques. The average increase in corneal refractive power for the early patients was 12.68 +/- 4.63 D with an average overrefraction of +0.45 +/- 5.60 D. The last half of the patients showed an average increase of 14.83 +/- 4.83 D, with an average overrefraction of -0.00 +/- 5.20 D. Visual acuity results in patients with traumatic cataracts have been the most satisfactory, with the majority of patients obtaining useful vision. It also appears that in patients with congenital cataracts, the younger the patient at the time of surgery, the greater the chance for a good visual result. Some improvement in vision has been seen in all of the children with successful grafts, even those beyond the age when amblyopia therapy would be considered to have some potential for therapeutic effect. By attaching the correction permanently on the eye, epikeratophakia facilitates the vigorous occlusion therapy required after cataract extraction in these children.

Compensating for thermally caused dimensional changes in the cryolathe

Dimensional changes in the Barraquer cryolathe , resulting from contraction due to refrigeration, can exert a considerable effect on the dimensions of the lathed lenticule . We have devised a simple technique for re- zeroing the cryolathe after freezing. This procedure negates the effects of the dimensional changes and thereby improves the accuracy of shaping the lenticules .

Refractive keratoplasty with hydrogel implants in primates

Histological examination of primate corneas with hydrogel lenses implanted in intrastromal pockets showed the corneal lamella posterior to the implant protruding into the anterior chamber; these eyes had shown no increase in anterior corneal curvature. In an attempt to obtain a dioptric increase without removing a corneal lamella, relaxing incisions were made in Bowman's layer followed by intrastromal implantation of hydrogel lenses. However, no refractive increase resulted, although six of the seven eyes were optically clear eight months postoperatively.

Histological study of epikeratophakia in primates

Two primate corneas were analyzed histologically 22 and 25 months after epikeratophakia surgery. Keratocytes had migrated from the host cornea into the lenticule through the surgical disruption of the host Bowman's membrane; the lenticule with the wider connection to the host cornea appeared to have more complete keratocyte repopulation. The lenticule stroma and host Bowman's membrane were continuously apposed along the interface, with a basement membrane-like material composed of collagen fibrils and amorphous matrix filling the interstices.

Corneal iron lines after refractive keratoplasty

Twenty-three of 55 patients who underwent refractive keratoplasty for the correction of aphakia were found to have a pigmented corneal epithelial line similar to the Fleischer ring. In 21 of 23 cases, the pigmented ring was adjacent to the superficial keratectomy scar that resulted from the keratorefractive operation. In two cases, the pigmented line was located central to the lamellar keratectomy scar. Histopathologic study demonstrated that the line was due to iron in the form of ferritin particles deposited within basal corneal epithelial cells. We believe that the steep anterior corneal curvature (average, 51.45 diopters) and changes in corneal topography at or central to the margin of the lamellar keratoplasty may lead to malapposition of the eyelid to the cornea and cause disturbances in the precorneal tear film. Iron from the tears may then be deposited in the corneal epithelium in areas of tear pooling.

Preliminary visual results of pediatric epikeratophakia

Although cataract surgery is feasible in children with unilateral cataracts, visual rehabilitation depends on optical correction and conscientiously maintained amblyopia therapy. Epikeratophakia for the correction of aphakia was performed in 47 children (50 grafts). Postoperative keratometry readings showed an average increase of 12.68 diopters in early patients. With new tissue-handling techniques, the last eight patients showed an average increase of +16.80 D, with -0.64-D overrefraction for emmetropia. Visual results indicate that epikeratophakia is an effective primary procedure for patients with unilateral traumatic cataracts and that it is superior to leaving vision uncorrected in children who have had unilateral congenital cataracts removed and are contact-lens intolerant. Further studies with younger patients will be necessary to define its role in neonates; however, individual results demonstrate that good vision is obtainable with epikeratophakia.

Corneal sensitivity after epikeratophakia

Corneal sensitivity was tested in 60 eyes of 30 patients who underwent unilateral epikeratophakia for the correction of aphakia (20 patients) or keratoconus (10 patients). Postoperative recovery time ranged from 2 months to 21 months (mean: 10 months). Our results indicate a relative hypesthesia of the epikeratophakia lenticule when compared with the peripheral host cornea and contralateral control cornea. However, corneal sensitivity tested in 11 patients with more than 1 year follow-up was increased compared with the sensitivity of 19 patients whose postoperative recovery was less than 1 year. Histopathologic findings in two lenticules from a nonhuman primate demonstrated sparse epithelial axon terminals. Host corneal nerves appear to innervate the lenticules by intraepithelial extension and by penetration of the superficial keratectomy scar.

On-lay lamellar keratoplasty for the treatment of keratoconus

On-lay lamellar keratoplasty was performed in 29 patients who had keratoconus. Twelve patients (greater than 1 year follow-up) showed an average decrease in keratometry of 9.13 +/- 2.02 (mean +/- SE) D; 6 patients (6-11 months follow-up) showed an average of 10.15 +/- 2.88 D. Visual acuity continued to improve for about one year after surgery. Six patients with best visual acuities of 20/20 before surgery showed an average visual acuity of 20/29 +/- 11 (mean +/- SD) 4 months after surgery; 4 with more than a year of follow-up achieved an average of 20/26 +/- 9. Four patients with 20/40 preoperatively achieved an average of 20/63 +/- 33 four months after surgery; 3 with more than a year of follow-up showed an average of 20/38 +/- 13. All patients with successful grafts can now tolerate contact lenses for daily wear.

Measurement of intraocular pressure after epikeratophakia

New forms of refractive surgery result in corneas that are nearly doubled in thickness. To test the reliability of standard instrumentation for the measurement of intraocular pressure in such eyes, the MacKay-Marg tonometer was used on eye bank eyes with and without epikeratophakia lenticules and/or bandage contact lenses. The Goldmann applanation prism in the hand-held Perkins portable tonometer and the MacKay-Marg tonometer were used to measure IOP in vivo in a primate eye with an epikeratophakia graft. Measurements were compared with actual intraocular readings from a transducer. The MacKay-Marg tonometer was reliable at pressures above 20 mm Hg; the extra thickness of the graft and/or the rigidity of the contact lens impaired the accuracy below 20 mm Hg. The Goldman tonometer was accurate over the entire range of pressures.

Epikeratophakia: the surgical correction of aphakia. Update: 1982

Sixty-five epikeratophakia procedures have been performed in 63 patients; visual acuity data have been tabulated on 31 patients with 4 to 30 months follow-up. Patients with more than a year of follow-up showed stable keratometry readings. Early patients achieved 70% of the predicted dioptric correction; more recent patients have achieved 87% with improvements in the lathing procedure, tissue handling, and surgical technique. Visual acuities improve with time. At any given time after surgery, acuities measured with a hard contact lens are better than those measured with spectacles; the decrease in spectacle acuity is probably a result of irregular refraction at the graft surface. The gap between contact lens and spectacle acuity decreases with time. A number of patients achieve postoperative visual acuities better than their preoperative acuities, and most achieve final spectacle acuities within a line or two of their preoperative acuities.

The cornea press: restoring donor corneas to normal dimensions and hydration before cryolathing

The shaping of swollen, abnormally hydrated donor corneal tissue on the cryolathe for use in keratorefractive surgery yields lenticules of insufficiently predictable dioptric powers. The irregular nature of this abnormal hydration precludes the use of mathematical compensating factors. To improve control over the shape of the lenticule, we have developed a cornea press which restores normal hydration and dimensions to the donor tissue by mechanical dehydration.

Update on keratophakia

Visual results are reported for 23 cases of keratophakia for the correction of surgical aphakia followed from 12 to 48 months. Seventeen of the 23 patients had final visual acuities of 20/40 or better. The average residual visual overrefraction was 1.87 +/- 1.79 diopters (mean +/- SE), with an increase of postoperative astigmatism of 1.70 +/- 1.33 diopters. Complications included interface deposits and peripheral epithelial deposits that did not interfere with vision. One patient had an anterior chamber penetration and subsequent penetrating keratoplasty, and two patients had lenticules removed because of corneal edema secondary to increased intraocular pressure. The major disadvantages of keratophakia are the complexity of the procedure and the time required (4 to 6 months) to achieve best-corrected spectacle visual acuity. However, because this procedure is extraocular, it is a better choice than secondary intraocular lens implantation for patients who are unilaterally aphakic and for the young patient.